High-strength, high-ductility noudlar iron, and transmission housing produced therefrom

ABSTRACT

An improved nodular cast iron is intended, despite having a high strength, to be ductile, to have good machining properties and to be weldable. It contains: 4 to 6 percent of silicon and a carbon content which has been reduced by an amount corresponding to the carbon equivalent, at most 0.03 percent of phosphorus and 0.2 to 0.5 percent of nickel. A casing (1) made from the nodular cast iron is welded to a toothed part (2) made from alloyed and/or case-hardened steel.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a nodular cast iron with alloyed-in silicon, as is customarily used for castings which are subject to high loads, for example the following DIN or SAE [in square parentheses] materials:

[0002] a) EN-GJS-400-16 [D4018],

[0003] b) EN-GJS-500-7 [D4512],

[0004] c) EN-GJS-600-3 [D5608],

[0005] d) EN-GJS-700-2 [D7003].

[0006] Of these, the first (a)) is still ferritic and therefore ductile, but its strength is only moderate. If the strength needs to be higher, materials b), c) and d) are used; in order, they range from ferritic-pearlitic to purely pearlitic, with a considerable drop in ductility and weldability. These materials have, to an extent which increases from a) to d), a multiphase (ferritic-pearlitic) basic structure of locally differing composition, a characteristic which is further amplified by the locally different cooling conditions which are inevitable during casting and leads to problems during the subsequent, in particular chip-forming, machining. Therefore, the use of these materials is restricted to particular applications.

[0007] In automotive engineering, weight reduction is a particular requirement, and in the case of castings, needs low wall thicknesses—i.e. a high strength—but also ductility and good machining properties. It is then necessary to use steel which, however, is approximately 10 percent heavier.

[0008] Although it is known from DE 196 36 808 C to use nodular cast iron with a high silicon content and a molybdenum content of 0.5 to 2 percent in automatic engineering for applications which require a high heat resistance, this material is very brittle and can only be machined with great difficulty.

[0009] Furthermore, U.S. Pat. No. 4,484,953 has disclosed a cast iron to which a high ductility and hardness are imparted by means of a complex heat treatment, which requires a very high nickel content. Both the nickel content and the heat treatment make this material disproportionately expensive.

[0010] Therefore, it is an object of the invention to improve nodular cast iron in such a way that, despite having a high strength, it is ductile, has good machining properties and can be welded.

SUMMARY OF THE INVENTION

[0011] A nodular cast iron according to the invention contains: 4 to 6 percent of silicon and a carbon content which has been reduced by an amount corresponding to the carbon equivalent, at most 0.03 percent of phosphorus, 0.2 to 0.5 percent of nickel, remainder iron and inevitable impurities. The high silicon content has the effect of stabilizing ferrite and also increases the strength. The carbon equivalent requires a corresponding reduction in the carbon content. Since phosphorus has an embrittling action, on account of the considerable solid-solution strengthening of the compounds which it forms, the phosphorus content should be kept as low as possible. Nickel also reduces the brittleness and increases the strength, and in this context the amounts required are so low that the additional costs entailed are negligible.

[0012] In this case, the strength is not obtained by means of a higher pearlite content, as is the case with nodular cast iron of the prior art, but rather by solid-solution strengthening. The combination of these measures results in a considerable increase in strength while maintaining a good ductility and weldability.

[0013] The quantitative ratios at which particularly good values have been achieved, are 4.1 to 4.5 percent of silicon and 0.25 to 0.35 percent of nickel. Limiting the copper content to below the normal amount intensifies the action of the measures according to the invention., since copper is a pearlite-forming agent.

[0014] Finally, the invention also relates to casings, in particular transmission casings, which consist of the nodular cast iron as claimed in one of the preceding claims. On account of the high strength of the nodular cast iron which has been improved in accordance with the invention, a transmission casing of this type can have thinner walls and can be of reduced weight. There is no restriction on the chip-forming machining which is required in order to form connection flanges and bearing seats.

[0015] Furthermore, a transmission casing made from the material according to the invention can be welded to a toothed part made from alloyed and/or case-hardened steel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the text which follows, the invention is explained with reference to tables and drawings, in which:

[0017]FIG. 1: shows a comparison of nodular cast iron according to the prior art with a nodular cast iron which has been improved in accordance with the invention,

[0018]FIG. 2: shows a section through a transmission casing according to the invention.

DETAILED DESCRIPTION

[0019]FIG. 1 compares the properties of the materials according to the prior art denoted a) to d) above, in the first four columns, with those of the material which has been improved in accordance with the invention, in the fifth column. The numbers and details speak for themselves and underline what has been stated in the introduction to the description.

[0020] The following example gives an analysis of the material in the fifth column of FIG. 1. the percentages given are the mean values for a plurality of castings formed from one batch: C Si Mn P S Cr Mo Ni Al - % % % % % % % % % X 3.142 −4.314 0.1922 0.0240 0.0074 0.0252 0.0024 0.3028 0.0035 Co Cu Nb Ti V W Pb Sn Mg - % % % % % % % % % X 0.0032 0.2276 0.0015 0.0093 0.0027 <0.0050 0.0017 0.0059 0.0293 As Zr Bi Ca Ca Sb Se Te B - % % % % % % % % % X 0.0049 0.0078 <0.0010 0.0040 0.0049 0.0028 <0.0010 0.0078 <0.0001 La N Fe - % % % X <0.0025 >0.0240 <91.6

[0021]FIG. 2 shows the casing, for example the casing of a transmission, denoted by 1, and a gearwheel, which is joined to the casing to form a single component and is denoted by 2. The casing 1 in this case consists of a nodular cast iron which has been improved in accordance with the invention. The gearwheel 2 is made from steel which has been case-hardened. The casing 1 has a first surface 3 which is to be welded, is normal to its axis and is adjoined by a cylindrical collar 4 which forms an outer cylindrical mating surface 5. The casing 1 may be provided, at a location of greater wall thickness, with a circumferential groove 6 which runs parallel to the first surface to be welded and is of rounded cross section. A second surface 7 which is to be welded, lying in a plane which is normal to the axis, and a cylindrical mating surface 8, which is seated on the cylindrical mating surface 5, are provided on the gearwheel 2.

[0022] The cylindrical mating surface 8 may be recessed at the transition to the second surface 7 which is to be welded, so that a gap 9 is formed. This facilitates production and makes it easier to push the gearwheel onto the collar 4, and also, if necessary, improves full root welding. The axis of rotation of the component is denoted by 18, and the welding head is denoted by 20. 

1. A nodular cast iron with alloyed-in silicon, comprising: a) 4 to 6 percent of silicon and a carbon content which is reduced by an amount corresponding to the carbon equivalent, b) at most 0.03 percent of phosphorus, c) 0.2 to 0.5 percent of nickel, d) remainder iron and inevitable impurities.
 2. The modular cast iron as claimed in claim 1, wherein the silicon content is 4.1 to 4.5 percent.
 3. The nodular cast iron as claimed in claim 1, wherein the nickel content is 0.25 to 0.35 percent.
 4. The nodular cast iron as claimed in claim 1, further including at most 0.3 percent of copper.
 5. A casing comprising a first part (1) of nodular cast iron as claimed in one of the preceding claims.
 6. The casing as claimed in claim 5, wherein the first part (1) is welded to a toothed part (2) made from one of alloyed steel and case-hardened steel. 